Then they do work. And I just tested it here on the very machine I am
typing from... and it does work with the release labels. Perhaps I
should make a second sources.list this way. To remove any confusion.

How did you test? I used 'apt-cache policy' after each change. When I
used codenames it was like the entry never existed.

apt.conf or preferences can work in that case?... In addition the same packages might appear in sid or experimental. ...#deb http://ftp.fr.debian.org/debian/ stable main contrib non-free ...#deb-src http://ftp.fr.debian.org/debian/ stable main contrib non-free...(Debian-User)